1. Field of Invention
The present invention relates in general to the electrical connector field. More particularly, the present invention relates to plastic land grid array (PLGA) modules and/or printed wiring boards (PWBs) incorporating contact pad preforms.
2. Background Art
Electrical connectors are in widespread use in the electronics industry. In many computer and other electronic circuit structures, an electronic module such as a central processor unit (CPU), memory module, application-specific integrated circuit (ASIC) or other integrated circuit, must be connected to a printed wiring board (PWB). Printed wiring boards are also known as printed circuit boards (PCBs). When populated with one or more electronic components, a printed wiring board is often referred to as a printed wiring board assembly (PWBA) or a printed circuit board assembly (PCBA). In connecting an electronic module to a PWB, a plurality of individual electrical contacts on the base of the electronic module must be connected to a plurality of corresponding individual electrical contacts on the PWB. This set of contacts on the PWB dedicated to contacting the electronic module contacts is known as a land grid array (LGA) site when a LGA connector is used to connect the electronic module to the PWB.
Rather than permanently soldering the electronic module contacts to the LGA site, it is desirable to use LGA connectors that allow the electronic module to be installed to and removed from the LGA site. LGA connectors provide the user with the flexibility to upgrade or replace electronic modules during the manufacturing cycle and in the field. LGA connectors are also known as interconnects. Generally, LGA connectors include “true” LGA connectors where both the PWB and the electronic module are mated with contacts through mechanical/pressure contact, as well as “hybrid” LGA connectors where the contacts are soldered to the PWB and make mechanical/pressure contact with the electronic module.
In general, LGA connectors provide electrical connections between two parallel electrical substrates in computing equipment through the use of an interposer. Typically, one of these substrates is a PWB and the other is an electronic module having either a ceramic laminate substrate or an organic laminate substrate. In the latter case of an electronic module having an organic laminate substrate, which is referred to herein as a plastic land grid array (PLGA) module, a plurality of individual LGA connectors extending through the interposer provide electrical connection between a plurality of corresponding individual electrical contacts on the PWB and on the PLGA module.
Most LGA connector technologies utilize individual electrical contacts that bear on wear resistant noble metal contact finishes to ensure interconnect robustness through application life. Although LGA connectors possess optimized surface finishes, various package process methods for PLGA modules limit the amount, thickness, and type of noble metal finish that can be directly plated onto the interconnect pads of PLGA modules. Electrolytic nickel and gold plating processes, for example, generally cannot be used on the interconnect pads of PLGA modules due toe electrical commoning requirements that drive wiring and process issues associated with the contact interface structure. It is typically preferable to use electrolytic nickel and gold plating processes to plate interconnect pads, if possible, because these processes can provide a thick nickel underplating (which may, for example, minimize diffusion of copper from the interconnect pad) and a thick, hard gold plating (which may, for example, provide wear and corrosion resistance).
Instead, electroless nickel and gold plating processes must generally be used on the interconnect pads of PLGA modules. Although these surface finish types offer some level of chemical nobility, thickness and hardness, the electroless nickel and gold plating processes are very restricted and provide narrow, if any, margin to ensure surface finish reliability in service. For example, electroless nickel and gold plating processes provide a relatively thin nickel underplating (which is less likely to sufficiently limit diffusion of copper from the interconnect pad) and a relatively thin, porous gold plating (which is less likely to provide sufficient wear and corrosion resistance). These limitations can be especially problematic in service applications that involve high plug/unplug counts and/or in service applications that are performed in corrosive operating environments.
Similar limitations also arise with respect to the other substrate connected by LGA connectors, i.e., the PWB. For example, the noble metal plating techniques typically used for plating interconnect pads on PWBs are impractical for a PWB that requires a subcomposite laminate construction and/or one or more double-sided LGA sites.
It should therefore be apparent that a need exists for an enhanced contact metallurgy construction for PLGA modules and/or PWBs, especially PWBs having a subcomposite laminate construction and/or a double-sided LGA site.